In recent years, following the high integration of semiconductor devices, wirings of circuits have been miniaturized, and the distance between the wirings has also been reduced. Especially, in the case of optical lithography for making micro-lithographic patterns having dimensions of 0.5 μm or less, the depth of focus becomes shallow, and hence the flatness of the image forming surface of the stepper is required. Therefore, it is necessary to flatten the surface of a semiconductor wafer, and as one of the flattening methods, a method is adopted in which the surface of a semiconductor wafer is polished by using a polishing apparatus.
Conventionally, a polishing apparatus of this kind includes a top ring and a turntable having a polishing cloth stuck to the upper surface thereof, and each of the top ring and the turntable is independently rotated at a rotational speed. Further, while a liquid (slurry) containing abrasive powder is poured onto the polishing pad stuck to the turntable, a semiconductor wafer, as a workpiece, which is set at the top ring, is pressed against the polishing pad, so that the surface of the semiconductor wafer is polished to a flat mirror surface.
The polishing speed of this kind of the polishing apparatus is varied by being influenced by variations in the surface state of the semiconductor wafer which have occurred in the previous process, and by the abrasion state of the polishing pad, and subtle changes of the slurry. If the semiconductor wafer is insufficiently polished, there arises a possibility that circuits are not insulated from each other and thereby short circuited with each other. Further, when the semiconductor wafer is excessively polished, the cross-sectional area of the wiring is reduced to cause such problems that the resistance value of the wiring is increased, and that the wiring itself is completely removed and thereby the circuit itself is not formed. For this reason, the polishing apparatus of this kind is provided with a polishing end point detection apparatus to detect an optimum polishing end point.
As a polishing end point detecting method of the polishing apparatus described above, there is known a method of detecting a change in the polishing friction force at the time when a different material begins to be polished with progress of the polishing (Japanese Patent Laid-Open No. 10-202523). A semiconductor wafer which is to be polished has a laminated structure formed of different materials including a semiconductor, a conductor, and an insulator, each of which has a different friction coefficient. Therefore, the method is configured to detect a change in polishing friction force at the time when a different material begins to be polished with progress of the polishing. In this method, the polishing process is ended at the time when a different material begins to be polished. Further, by detecting a change in polishing friction force at the time when the surface of a semiconductor wafer is changed from an uneven state to a flattened state, the polishing apparatus can also detect that the surface of the semiconductor wafer is flattened.
Here, a change in polishing friction force is detected as follows. Since polishing friction force acts on a position deviated from the rotation center of the turntable, the polishing friction force acts as load torque on the rotating turntable. For this reason, the polishing friction force can be detected as torque acting on the turntable. When means for rotationally driving the turntable is an electric motor, the load torque can be measured as a current flowing into the motor. For this reason, the polishing end point is detected in such a manner that the motor current is monitored with an ammeter and that the measured result is subjected to a suitable signal processing.
FIG. 10 shows a configuration example of a method for detecting a polishing end point on the basis of a change in the current inputted into a drive motor. An electric motor 500 is driven by an AC commercial power supply 512 via an inverter apparatus 510. In the inverter apparatus 510, power from the AC commercial power supply 512 is converted into DC power by a convertor unit 514, so as to be accumulated in a condenser 516. The DC power is inversely converted into AC power of an arbitrary frequency and an arbitrary voltage by an inverter unit 518, and the converted AC power is supplied to the electric motor 500 via a three-phase cables 520. The three-phase cables of the inverter apparatus 510, which supply the AC power to the electric motor 500, are respectively connected to three-phase field windings of the electric motor 500. A current convertor (CT) 522 is provided at the cable of one phase, for example, the cable of the V-phase, of the three-phase cables 520 which supply the AC power to the electric motor 500 to detect the motor current. As the value of the motor current flowing through the current supply line connected to the electric motor 500, the value of current flowing through the V-phase is detected by an ammeter 524 and is sent to end point detection means of a control circuit of a polishing apparatus (not shown), so that the polishing end point is determined on the basis of a change in the value of the detected current.
In recent years, as semiconductor devices have become more highly integrated, wirings of circuits have been becoming more miniaturized, and the distance between the wirings have also been reduced more than before. Therefore, it is desired to further improve the flatness of a semiconductor wafer. However, the above-described method, in which the value of current flowing through one phase is detected by an ammeter, and in which the polishing end point is determined on the basis of a change in the detected current value, is not sufficient in order to improve the flatness of a semiconductor wafer more than before.
Further, in the conventional technique described above, the polishing end point is detected in such a manner that the current of one phase (for example, V-phase) of the three phases of the electric motor is measured, and that a change in the torque of the electric motor is detected on the basis of a change in the detected current. In practice, however, each phase current of the electric motor can be varied. In addition, the current of each phase of the electric motor is not changed in a manner that the current value of a specific phase is always increased or decreased, but there is a possibility that the current of the electric motor is variously changed due to variations between the electric motors and due to variations between the polishing apparatuses.
In this situation, when the polishing end is detected by measuring current of specific one phase of the electric motor, the detected current is variously changed, which results in a possibility that, when a change in the torque of the electric motor is detected, the detected value is also variously changed.